Nitric oxide generated from intact mitochondria was detected by nitronyl nitroxide. Incubation of mitochondrial particles with nitronyl nitroxide in the presence of substrates resulted in the reduction of nitroxide. Supernatants were treated with potassium ferricyanide to regenerate the nitroxide signal. We used the cationic nitronyl nitroxide in order to increase the intramitochondrial concentration of the trap. Preliminary results based on oxygen consumption measurements indicate that cationic nitronyl nitroxide is able to trap the nitric oxide produced in intact mitochondria.